Question

I’m looking to replace some snprintf calls with stringstreams. I need to ensure the output is identical.

given long l = some_input_var;

do

std::ostringstream str;
str << l;
return str.str();

and

buf str[24];
snprintf(str, 24, "%ld", l);
return std::string(str);

return identical strings?

What about for int and "%d" or double and "%f" ?

Answer 1

Yes. But it’s a long story: iostreams formatting works via the stream’s codec’s formatting facet. The details are fairly involved; see C++11 27.7. The default facet’s conversion uses sprintf for input formatting and the strtoull-type functions for output; see 22.4.2.1 for details.

Also and similarly, the new <string> conversion functions std::to_string use the default sprintf format, and the various std::stoul-like functions use the C library’s strtoul-like functions.